Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Freezing Boiling Density Lbs./ Point .degree.F. Point .degree.F. U.S. Gal. ______________________________________ Ethylbenzene -139.0 277.0 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources may vary quite widely in composition but will usually be in the range 10 to 32 wt.% ethylbenzene with the balance, xylenes, being divided approximately 50 wt.% meta, and 25 wt.% each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes, as by OCTAFINING.
In a typical plant for utilization of Octafining, a mixture of C.sub.8 aromatics is introduced to an ethylbenzene tower wherein the stream is stripped of a portion of its ethylbenzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive "superfractionation". Ethylbenzene is taken overhead while a bottom stream, consisting principally of xylenes, together with a significant amount of ethylbenzene, passes to a xylene splitter column. The bottoms from the xylene splitter constituted by o-xylene and C.sub.9 aromatics passes to the o-xylene tower from which o-xylene is taken overhead and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell, et al., U.S. Pat. No. 3,662,013 dated May 9, 1972.
Because it's melting point is much higher than that of the other C.sub.8 aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can be recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product constituted by C.sub.8.sup.+ hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethylbenzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining was originally unique in its ability to convert ethylbenzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C.sub.8 aromatic fractions. As will be seen from the table of properties above, the boiling point of ethylbenzene is very close to those of p and m-xylene. Complete removal of ethylbenzene from the charge is impractical. The usual expedient for coping with the problem was an ethylbenzene separation colum in the isomerizer-separator loop when using catalyst other than those characteristic of Octafining. It will be seen that Octafining does not need this expensive auxiliary to prevent build up of ethylbenzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethylbenzene.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane and benzene and hydrocracking of alkyl cyclohexanes.
A significant improvement arose with the introduction of catalysts such as zeolite ZSM-5 combined with a metal such as platinum as described in Morrison U.S. Pat. No. 3,856,872. At temperatures around 700.degree.-800.degree. F., ethylbenzene is converted by disproportionation over this catalyst to benzene and diethylbenzene. At higher temperatures and using a zeolite ZSM-5 catalyst of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in U.S. Pat. No. 4,188,282.
These developments permit upgrading of Octafining reactors by substitution of the improved (ZSM-5) catalyst.
In the known processes for accepting ethylbenzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to other compounds to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethylbenzene conversion against the disadvantages of xylene loss by disproportionation and the like.
A further advance in the art is described in patents to Morrison and Tabak directed to various techniques for reducing acid activity of zeolite ZSM-5 catalyst and use of such low activity catalysts for xylene isomerization concurrently with ethylbenzene conversion at temperatures upwards of 800.degree. F. One such patent is U.S. Pat. No. 4,163,028 which discloses xylene isomerization and ethylbenzene conversion at high temperature with ZSM-5 of very high silica/alumina ratio whereby the acid activity is reduced.
The inventions of those patents are predicated on discovery of combinations of catalyst and operating conditions which decouples ethylbenzene conversion from xylene loss in a xylene isomerization reaction, thus permitting feed of C.sub.8 fractions which contain ethylbenzene without sacrifice of xylenes to conditions which will promote adequate conversion of ethylbenzene. These results are obtained by use of a catalyst characterized by zeolite ZSM-5 substantially reduced in activity as by dilution, steaming, very high silica/alumina ratio, base exchange with alkali metal, coking or the like. At the high temperatures of 800.degree.-1000.degree. F., the zeolite of reduced activity exhibits effective power for isomerization of xylene and for splitting off alkyl side chains of two or more carbon atoms from single ring aromatics at long on-stream periods. The disproportionation activity of the zeolite is severely depressed by the reduced acid activity, resulting in low losses of xylene by that mechanism. That lack of disproportionation activity impairs the capacity of the catalyst to handle trialkyl aromatics of nine or more carbon atoms, e.g. trimethylbenzene, as practiced in some processes. It thus becomes necessary to remove from the recycle stream those components having more than eight carbon atoms to avoid excessive build-up in the system of C.sub.9 and higher. The catalyst also has the capacity to crack paraffins in the charge to lower boiling compounds readily removable from recycle streams by fractionators normally present in the p-xylene recovery/isomerizer loop.
By reason of this combination of activities, the catalyst may be used in a system charging reformate without removal of paraffin hydrocarbons as described in U.S. Pat. No. 4,211,836.
In prior copending application Ser. No. 140,346 filed Apr. 4, 1980, it is shown that such processes charging unextracted reformate are improved by the addition of paraffins having no more branching than a single methyl group on any carbon atom of the paraffin chain, e.g., n-nonane. The entire disclosure of prior application Ser. No. 140,346 is incorporated herein by this reference.